The microbial world is hugely diverse owing to its 3.7 billion years of evolution, which provides for both the opportunity of undiscovered metabolic capacity, including that for pollutant degradation, and the challenge of detecting, recovering, and characterizing this activity. While microbes have been exposed to small amounts of dioxins over millennia due to natural events such as forest fires, it is only in the last century or so that dioxins have become an abundant food source in nature due to anthropogenic activity. This may explain why there are only a few well characterized microbial strains with the ability to mineralize dioxins. We propose to characterize the microbial response to dioxins in pure cultures and microbial communities to understand the limitations on environmental detoxification of this hazardous class of compounds. Our long term goal is to develop microbes for enhanced biodegradation, to identify molecular markers to aid in site assessment, and to develop models for prediction of biodegradation at contaminated sites. Our Specific Aims in the present proposal are: (1) to elucidate the physiology, biochemistry, and genetics of chlorinated dioxins degradation in order to understand and improve the remediation of these highly toxic pollutants, (2) to assess (meta)genome-wide responses and metabolic capabilities by conducting physiogenomic studies of microorganisms and active microbial communities in response to chlorinated dioxins including those from environmentally important geosorbents, and (3) to explore and recover nature's catalytic diversity with the goal of developing a comprehensive profile of microbial community metabolic capabilities for degradation and/or mineralization of chlorinated dioxins and dioxin-like compounds in the environment. We propose to use enrichments to help identify the functionally active populations and hence genes, to use stable isotope probing to recover DNA from the active degrading populations, to use metagenomic and metatranscriptomic libraries and the novel technology of massive parallel RNA bait capture of target DNA and RNA to recover the full genes and operons, to use this information to understand the biochemistry of dioxins degradation, and to use transcriptomic and biochemical analyses of a model dioxin degrading bacterium to understand the total response of the organism to the various pressures involved during the degradation process (i.e. substrate and metabolite toxicity, metabolic fluxes, etc.).